Fabricating a gallium nitride device with a diamond layer

ABSTRACT

In one aspect, a method includes fabricating a device. The device includes a gallium nitride (GaN) layer, a diamond layer disposed on the GaN layer and a gate structure disposed in contact with the GaN layer and the diamond layer. 
     In another aspect, a device includes a gallium nitride (GaN) layer, a diamond layer disposed on the GaN layer and a gate structure disposed in contact with the GaN layer and the diamond layer.

BACKGROUND

Gallium Nitride (GaN) has electrical and physical properties that makeit highly suitable for high frequency (HF) devices such as microwavedevices. The HF devices produce a high amount of heat requiring a heatspreader to be attached to the HF devices to avoid device failure. Onesuch heat spreader is diamond. A hot filament chemical vapor deposition(CVD) process has been used to form diamond that is used on GaN layers.Generally, these diamond layers are not deposited directly onto the GaNlayers but onto some other material (e.g., silicon, silicon carbide, andso forth) that is eventually disposed with the GaN layer.

SUMMARY

In one aspect, a method includes fabricating a device. The deviceincludes a gallium nitride (GaN) layer, a diamond layer disposed on theGaN layer and a gate structure disposed in contact with the GaN layerand the diamond layer.

In another aspect, a device includes a GaN layer, a diamond layerdisposed on the GaN layer and a gate structure disposed in contact withthe GaN layer and the diamond layer.

In a further aspect, a method includes disposing a diamond layer onto afirst surface of gallium nitride (GaN), removing a portion of thediamond layer exposing the first surface of the GaN and forming a gatestructure in contact with the first surface of the GaN and the diamondlayer.

DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram of an example of a Gallium Nitride (GaN) layer witha first diamond layer and a second diamond layer.

FIG. 1B is a diagram of another example of the GaN layer with the firstdiamond layer and the second diamond layer.

FIG. 2 is a flowchart of an example of a process to fabricate the GaNlayer with the first diamond layer and the second diamond layer.

FIGS. 3A to 3D are diagrams corresponding to the process of FIG. 2.

FIG. 4 is a flowchart of another example of a process to fabricate a GaNlayer with the first diamond layer and the second diamond layer.

FIGS. 5A to 5H are diagrams corresponding to the process of FIG. 4.

FIG. 6 is a flowchart of an example of a process for depositing diamondon another surface.

FIGS. 7A to 7F are diagrams corresponding to the process of FIG. 6.

FIG. 8 is a flowchart of another example of a process for depositingdiamond on another surface.

FIGS. 9A to 9D are diagrams corresponding to the process of FIG. 6.

FIG. 10 is an example of a device with diamond layers.

FIG. 11 is another example of a device with diamond layers.

FIG. 12 is a graph depicting thermal performance with diamond coatings.

DETAILED DESCRIPTION

Hot filament chemical vapor deposition (CVD) processes have been used toform diamond layers of less than 1 mil that are used on gallium nitride(GaN) layers. To be effective as a heat spreader, diamond layers must begreater than 2 mils. Moreover, the hot filament CVD process by its verynature produces a blackish-color diamond which is contaminated withmaterial used in the hot filament CVD process such as tungsten, forexample. In general, these “dirty” diamond layers that are produced havea lower thermal conductivity than pure diamond. In general, the thermalconductivity of diamond layers using the hot filament CVD process isabout 800 to 1000 Watts/meter-Kelvin (W/m-K).

A microwave plasma CVD process has been known to produce much thickerdiamond layers on the order of 4 mils or greater at a much faster ratethan the hot filament CVD process. Moreover the diamond layers are purerthan the hot filament CVD process producing diamond layers having athermal conductivity greater than 1500 W/m-K. In one example, thethermal conductivity of diamond produced using the microwave plasma CVDprocess is twice the thermal conductivity of diamond produced using thehot filament process. However, the CVD processes including the microwaveplasma CVD process is relatively unknown with respect to directdeposition onto GaN. For example, the deposition of diamond using hotfilament CVD is typically done onto some other material (e.g., silicon,silicon carbide, and so forth) that is eventually is disposed with theGaN layer. Since the deposition of diamond directly onto to GaN usingthe microwave plasma CVD process is relatively unknown, the costs ofdeveloping and testing a reliable and successful processes to depositdiamond directly onto the GaN is extremely expensive. One way around thecost and expense of developing a process to deposit diamond directlyonto GaN, is to deposit diamond using the microwave plasma CVD processonto an inferior diamond layer that was fabricated using the hotfilament CVD, for example.

As used herein GaN layers may include pure GaN, doped GaN or GaNcombined with other elements (e.g., AlGaN) or any combination thereof.Silicon substrates may include pure silicon, doped silicon, silicondioxide, silicon carbide or any combination of silicon with otherelements or any combination thereof.

Referring to FIGS. 1A and 1B, in one example, a structure 10 for use informing a device (e.g., a high frequency device, a high electronmobility transistor (HEMT), a microwave device and so forth) includes asecond diamond layer 12, a first diamond layer 14 adjacent to the seconddiamond layer and a GaN layer 16 adjacent to the first diamond layer. Inthis configuration, heat produced by GaN layer 16 pass through a heatspreader formed by the first and second diamond layers 12, 14. Inanother example, a structure 20 uses to form a device (e.g., a highfrequency device, a HEMT transistor, a microwave device and so forth) issimilar to the structure 10 but includes an interlayer 22 between thefirst diamond layer and the GaN layer 16. The interlayer 22 is neededbecause the fabrication of diamond directly onto GaN is not easy processmuch less predictable or consistent. The interlayer 22 may be simply anadhesive holding the first diamond layer 14 to the GaN 16 or asilicon-type structure onto which diamond may easily be disposed.Sometimes the interlayer 22 has a thermal conductivity less than that ofthe diamond layers 12, 14 so that it holds heat more; or put anotherway, the heat transference from the GaN layer 16 is impeded by theinterlayer 22. Thus, minimizing the interlayer 22 or not having theinterlayer at all as in the structure 10 is preferred.

Referring to FIGS. 2 and 3A to 3D, one process to fabricate a GaN layerwith a first diamond layer and a second diamond layer is a process 100.The hot filament CVD process is used to deposit a first diamond layer 14(e.g., a layer of 5 to 20 microns thick) onto a silicon-on-insulator(SOI) substrate 122 (102) (FIG. 3A). The insulator (not shown) (e.g.,silicon dioxide) is removed from the SOI substrate 122 leaving a siliconsubstrate 122′, for example (104) (FIG. 3B). The microwave plasma CVD isused to deposit a second diamond layer 12 onto the first diamond layer14 (108) (FIG. 3C). GaN is grown onto the remaining SOI substrate, thesilicon substrate 122 (112) (FIG. 3D).

Referring to FIGS. 4 and 5A to 5H, another process to fabricate a GaNlayer with a first diamond layer and a second diamond layer is a process200. GaN 16 is grown on a first substrate 230 (202) (FIG. SA). In oneexample, the first substrate may be silicon carbide, silicon orsapphire. A silicon layer 232 (e.g., silicon, silicon carbide and soforth) is disposed onto the GaN (204) (FIG. 5B). In one example, thesilicon layer 232 is attached to the GaN 16 using an adhesive. Inanother example, the silicon layer 232 is grown onto the GaN 16. Inother examples, other materials such as glass may be used instead of thesilicon layer 232. The first substrate 230 is removed (208), forexample, through etching leaving a GaN/silicon structure 250 (FIG. 5C).A hot filament CVD is used to deposit a first layer of diamond 14 onto asecond substrate 234 (212) (FIG. 5D). For example, the second substrate234 is a silicon substrate 500 microns thick. A microwave plasma CVDprocess is used to deposit a second diamond layer 12 onto the firstdiamond layer 14 (218) (FIG. 5E). The second substrate 234 is removed(218), for example, through etching (FIG. 5F). The first and seconddiamond layers 12, 14 are attached to the GaN/silicon structure 250(224) (FIG. 5G). For example, the first diamond layer 14 is attached tothe GaN 16 using an adhesive. The silicon layer 232 is removed (228),for example, through etching (FIG. 5H).

Referring to FIG. 6 and 7A to 7F, a further process to fabricate a GaNlayer with diamond layers is a process 300. Process 300 is similar toprocess 200 except a third diamond layer 316 is disposed on a first GaNsurface 302 (e.g., a top surface) (FIG. 7F) opposite a second GaNsurface 304 (e.g., a bottom surface) (FIG.7F) that has the first andsecond diamond layers 14, 12. For example, processing blocks 202, 204and 208 are performed as in process 200. In particular, the GaN 16 isgrown on the first substrate 230 (202) (FIG. 7A), the silicon layer 232is disposed onto the GaN 16 (204) (FIG. 7B); and the first substrate 230is removed (208), for example, through etching leaving the GaN/siliconstructure 250 (FIG. 7C).

The silicon/GaN structure 250 is immersed in a solution and subjected toultrasound (302). By treating the surface prior to deposition (e.g., aprocessing block 314), the diamond layer 316 has a better chance offorming on the GaN 16 during deposition. In one example, the solution isan isopropyl alcohol solution that includes diamond particles (e.g.,nano-diamond particles (10⁻⁹ m)).

The third diamond layer 316 is disposed on the silicon/GaN structure 250(314) (FIG. 7D). For example, the microwave plasma CVD process is usedto deposit the third diamond layer 316 onto the GaN 250 at temperaturesfrom about 600° C. to about 650° C. The silicon layer 232 is removed(228), for example, through etching (FIG. 7E).

The first and second diamond layers 14, 12, formed using process blocks212, 214 and 218, for example, are attached to the remaining GaN/diamondstructure to form a diamond/GaN/diamond/diamond structure 360 (334)(FIG. 7F). For example, the first diamond layer 14 is attached to theGaN 16 using an adhesive. The first diamond layer 14 is attached to thesecond surface 304 opposite to the first surface 302 disposed with thethird diamond layer 316. By having a diamond layer 316 disposed onopposite surfaces from the diamond layers 12, 14, heat is moreeffectively pulled away from devices formed from thediamond/GaN/diamond/diamond structure 360.

Referring to FIG. 8 and 9A to 9D, a still further process to fabricate aGaN layer with diamond layers is a process 370. A silicon carbide/GaNstructure 380 (FIG. 9A) that includes a GaN layer 16 and a siliconcarbide layer 382 disposed with the second surface of the GaN 16. Thesilicon carbide/GaN structure 380 is immersed in an isopropyl alcoholsolution with nano-diamond particles (e.g., a solution used inprocessing block 312) and an ultrasound is performed (372). A thirddiamond layer 316 is disposed on the GaN 16 (374) (FIG. 9B). The siliconcarbide layer 382 is removed, for example, through etching (376) (FIG.9C). The first and second diamond layers 14, 12 are formed usingprocessing blocks 212, 214, and 218, for example. The first and seconddiamond layers 14, 12 are attached to the GaN/diamond 350 to form thediamond/GaN/diamond/diamond structure 360 (334) (FIG. 9D).

Referring to FIG. 10, the diamond/GaN/diamond/diamond structure 360 maybe used to fabricate devices such as a high frequency device, a highelectron mobility transistor (HEMT), a microwave device and so forth.For example, the diamond layer 316 may be integrated directly into thedevices and used not only to remove heat but function as a dielectric,for example, used in capacitance. For example, the dielectric constantof diamond is about 5.7 which is close to the dielectric constant ofabout 7 for silicon nitride films commonly used in GaN devices; however,diamond films have a greater thermal conductivity than the siliconnitride films. In some examples, portions of the diamond layer 316 areremoved (e.g., using oxygen plasma) and the surface 302 of the GaN 16becomes exposed.

In one example, a device 400 (e.g., a HEMT device) includes a source404, a drain 406 and a gate 408 (e.g., a T-Gate) that are deposited in ametallization step onto to the surface 302 of the GaN layer 16. The gate408 is formed in the diamond layer 316 after removal of portions of thediamond layer thereby exposing the GaN. In this example, the removal ofportions of the diamond layer 316 splits the diamond layer into twodiamond layers 316 a, 316 b each having a width W. In thisconfiguration, the diamond layers 316 a, 316 b may function as adielectric layer and a heat spreader by removing the heat away from thegate 408. In some examples, the widths of the diamond layers 316 a, 316b may not be equal. In one example, portions of the gate 408 areadjacent to and in contact with the diamond layers 316 a, 316 b andother portions of the gate 408 form gaps 410 a, 410 b (e.g., air gaps)between the gate and the diamond layers 316 a, 316 b. In one example,gate 408, the gaps 410 a, 410 b, the diamond layer 316 a, 316 b formcapacitance structures. One of ordinary skill in the art would be awareof several methods to form these gaps 410 a, 410 b. For example, priorto metallization to form the gate 408, a material (e.g., photoresist)may be on the surface of the diamond layer 316. After the gate 408 isformed, the material is removed forming the gaps 410 a, 410 b. In otherexamples, the device 400 does not include gaps 410 a, 410 b so that thegate 408 is directly on the surface of the diamond layers 316 a, 316 b.In still further examples, other materials may fill gaps 410 a, 410 bthat may or may not contribute to capacitance.

Referring to FIGS. 11 and 12, a device 400′ is similar to the device 400with the GaN layer 16 including an AlGaN layer 412 and a pure GaN layer416. Other GaN-type materials may be added to the GaN layer 416 than theAlGaN 412. The GaN layer 416 may also be replaced with doped GaN orother GaN-type materials. The third diamond layers 316 a, 316 b are usedto significantly reduce temperatures at the gate 408 by spreading theheat away from the gate. A graph 500 depicts the effects of heat as afunction of the width, W, of the diamond layer 316 a or 316 b using thedevice 400′. A distance, D, between the gate 408 and the source 404 is1.875 microns and a distance, G, between the diamond layers 316 a, 316 bis 0.25 microns. A curve 502 represents a 0.05 micron layer of diamondand a curve 504 represents a 0.25 micron layer of diamond. The 0.25micron diamond coating allows a 20% increase in output power and reducesthermal resistance by 15% (>25° C. at 5 W/mm) than not having a diamondlayers 316 a, 316 b. The 0.05 micron diamond coating reduces thermalresistance by 10% (>25° C. at 5 W/mm) than not having a diamond layers316 a, 316 b.

The processes described herein are not limited to the specificembodiments described herein. For example, the processes are not limitedto the specific processing order of the process steps in FIGS. 2, 4, 6and 8. Rather, any of the processing steps of FIGS. 2, 4, 6 and 8 may bere-ordered, combined or removed, performed in parallel or in serial, asnecessary, to achieve the results set forth above.

While the invention is shown and described in conjunction with aparticular embodiment having an illustrative product having certaincomponents in a given order, it is understood that other embodimentswell within the scope of the invention are contemplated having more andfewer components, having different types of components, and beingcoupled in various arrangements. Such embodiments will be readilyapparent to one of ordinary skill in the art. Other embodiments notspecifically described herein are also within the scope of the followingclaims.

1. A method, comprising: fabricating a device comprising: a galliumnitride (GaN) layer; a first diamond layer disposed on the GaN layer;and a gate structure disposed in contact with the GaN layer and thefirst diamond layer, wherein the fabricating comprises: depositing afirst diamond layer onto a first surface of the GaN; and disposing asecond diamond layer onto a second surface of the GaN layer opposite thefirst surface of the GaN layer.
 2. The method of claim 1 wherein thedepositing comprises depositing the diamond layer onto the GaN layerusing a microwave plasma chemical vapor deposition (CVD).
 3. The methodof claim 1 wherein the depositing comprises depositing a diamond layerthat is greater than 1,000 Angstroms.
 4. The method of claim 1 whereindisposing a second diamond layer onto a second surface of the GaN layeropposite the first surface of the GaN layer comprises attaching to thesecond surface of the GaN layer a second diamond layer having a firstthermal conductivity and a third diamond layer disposed thereto having asecond thermal conductivity greater than the first thermal conductivity.5. The method of claim 1 wherein fabricating a device comprising the GaNlayer comprises fabricating a device comprising a GaN layer comprisingat least one of undoped GaN, doped GaN or GaN combined with anotherelement.
 6. The method of claim 1 wherein fabricating a device comprisesfabricating one of a high frequency device, a high electron mobilitytransistor (HEMT) or a microwave device.
 7. A method comprising:disposing a diamond layer onto a first surface of gallium nitride (GaN);removing a portion of the diamond layer exposing the first surface ofthe GaN; forming a gate structure in contact with the first surface ofthe GaN and the diamond layer; and attaching a second diamond layer to asecond surface of the GaN opposite the first surface of the GaN.
 8. Themethod of claim 7, further comprising fabricating at least one of a highfrequency device, a high electron mobility transistor (HEMT) or amicrowave device from the diamond/GaN structure.
 9. The method of claim7 wherein removing a portion of the diamond layer exposing the GaNsurface comprises removing a portion of the diamond layer using oxygenplasma.
 10. The method of claim 7 wherein disposing a diamond layercomprises depositing a diamond layer.
 11. The method of claim 10,further comprising performing an ultrasound on the GaN immersed in asolution comprising nano-diamond particles prior to depositing thediamond layer.
 12. The method of claim 7, further comprising: growingGaN on a first substrate; disposing a layer of material onto the GaN;and removing the first substrate.
 13. The method of claim 12 whereingrowing GaN on a first substrate comprises growing GaN on one of siliconcarbide, silicon or sapphire.
 14. The method of claim 12 whereindisposing a layer of material onto the GaN comprises disposing one of asilicon layer or a glass layer.
 15. The method of claim 12, furthercomprising removing the layer of material to form a diamond/GaNstructure.